Polyesters are widely used in the manufacture of fibers, molded objects, films, sheeting, food trays, and food and beverage containers. These polymers are generally made by melt phase polymerization reactions which are well known in the art. It is common practice in the art to pelletize the generated polyester for subsequent extrusion or molding operations which require remelting of the pelletized polyester. However, during the processing of polyesters in the melt phase, certain undesirable by-products are formed. One such by-product is acetaldehyde which is continually formed as a by-product during the polymerization and subsequent melt processing of polyesters. Acetaldehyde is known to contaminate food or beverage products when it is present in a food or beverage container. Therefore, it is desirable to produce molded polyester containers having an acetaldehyde content at a low or zero level.
A three stage process has been generally used to produce polyester polymers. This process typically involves the preparation of a low molecular weight polyester precursor by melt phase polymerization techniques that are well known in the art. This precursor is then pelletized and solid-state polymerized. Typically during the solid-state polymerization, an inert gas is used to strip acetaldehyde and other by-product from the pellets, producing pellets with a low acetaldehyde of about 1 ppm or less. However, when the pellets are remelted and formed into a molded product, the acetaldehyde content in the polyester increases to an undesirable level of from about 8 ppm to about 10 ppm or more. Therefore, a more efficient method of producing a polyester article with low acetaldehyde content has been desired.
U.S. Pat. No. 5,597,891 describes an improved process for lowering the acetaldehyde content of molten polyethylene terephthalate (PET) to levels suitable for direct use in food packaging articles by mixing an inert gas with the molten polyester. In this process, devolatilization occurs through venting of a continuous screw conveyor, such as a single or multi-screw extruder. U.S. Pat. No. 4,734,243 describes an injection molding machine for plastics provided with a plasticization device which consecutively plasticizes material and feeds a plurality of injection devices in sequence. German Patent DE 19505680 and U.S. Pat. No. 5,656,221 describe a process for the production of bottle preforms from a melt whereby an inert gas is admitted into the continuous flow of the polyester melt. In this process, devolatilization occurs in a vented extruder. U.S. Pat. No. 5,656,719 describes the direct production of molded packages that imparts no taste from thermoplastic polyesters. An inert gas is dispersed into the polyester by means of stationary pipeline mixers, and the polyester is degassed in an enlarged pipe segment under vacuum.
Japanese Application Hei 5-315154 describes a method and apparatus for water injection, dispersion, bubbling, and degassing whereby volatiles present in the said polymer are vaporized and removed. European Patent 0525748A1 describes a method and apparatus for removal of hydrocarbons from polymer slurries. U.S. Pat. No. 3,470,070 describes a similar procedure wherein hexane is removed from hydrogenated polybutadiene. U.S. Pat. No. 3,458,494 describes a flash tank apparatus for removing cyclohexane from polyethylene. The effluent is flashed through a nozzle into the flash tank at which point it contacts heat exchanger plates and the solvent evaporates. U.S. Pat. No. 3,476,736 describes solvent recovery in polyolefins via a vented extruder. U.S. Pat. Nos. 5,380,822 and 5,350,813 describe processes for removing residual monomer in polymers by injecting water or a suitable, condensable fluid into the melt and passing the effluent through a flash tank. U.S. Pat. No. 5,543,495 describes a process whereby an inert gas is injected into a condensation polymer melt at elevated temperature. U.S. Pat. No. 3,989,677 describes an improved process for the production of polyamides by treating the melt with water or steam and devolatilizing in a vented extruder. U.S. Pat. No. 4,728,701 describes the use of flash tank devolatilization for the removal of solvent and monomer from acrylate polymers after polymerization. U.S. Pat. No. 4,294,652 describes an improved devolatilizer using multiple tanks or stages.
Johnson, John M. (Popular Plastics and Packaging, April 1994, p.59) discusses various means for devolatilization including vented extruders and flash tanks. This method is applied to the processing of pelletized polymer rather than a melt to mold process wherein a polyester is polymerized and then devolatilized directly from the melt, before being molded into a desired shape. Mack, M. H. et al (Proceedings of ANTEC '93, SPE, p. 1060) describe the effect of various stripping agents on the removal of residual hydrocarbon in LDPE using a vented single-screw extruder. Meister, B. J., et al (Ind. Eng. Chem. Res 28, 1989, p.1659) disclose the devolatilization performance of a flash tank in a commercial polystyrene line. No stripping agent was taught.
The present invention provides an improved apparatus and a method of forming polyesters articles having a low acetaldehyde content. In the practice of the present invention, molten polyester is prepared by continuously reacting polyester precursors and injecting an acetaldehyde stripping agent which is inert to the polyester into the melt under pressure. The polyester is then devolatilized in a flash tank under vacuum and molded directly from the melt into shaped articles. The apparatus and method of the present invention avoids the costly steps of reprocessing pelletized polyester, and forms articles with superior qualities such as better color, enhanced molecular weight, and fewer physical defects, as well as having a low acetaldehyde content.